Medicare managed care in the US has grown from less than 2.6% in 1986 to a projected 20% by the year 2000. Despite this dramatic growth and strong support among policy makers to expand Medicare managed care, major gaps in our knowledge of the effects of managed care on the Medicare population remain that are hampering the development and implementation of policy. The major barrier to research has been the lack of data on utilization of Medicare managed care enrollees after they join an HMO. This study exploits a newly constructed database that includes such data. Specifically the database covers monthly enrollment status and all of the inpatient utilization of all Medicare beneficiaries in California including both HMO and FFS enrollees from 1991-1996. (If this grant is funded, we will ai,,,d 1997-1999 to the existing sir years). It will facilitate, for the first time, a comprehensive assessment of what happens to Medicare beneficiaries before and after they join an HMO. Preliminary descriptive analyses indicate that the sample is large and rich enough to support descriptive and multivariate analyses on many important research and policy questions that previous research has been unable to address. We focus on research questions that remain controversial in the scientific literature as well as analyses designed to contribute to the current debate on Medicare managed care policy. The specific aims of this project are: 1. To estimate the extent and persistence of selection bias from enrollment and disenrollment in Medicare Risk HMOs and to evaluate HCFA's proposed risk-adjustment methodology (PIPDCGs) for payment. 2. To investigate the effects of beneficiary characteristics (including predicted expenditures), plan characteristics and market structure on disenro1lment from Medicare Risk HMOs. 3. To compare and evaluate alternative measures of Medicare Risk HMO plan quality based on administrative data.